1. Field of the Invention
The present invention relates to charging devices used in image forming devices such as copying machines and printers for electrically charging surfaces of electrostatic latent image support members.
2. Description of the Related Art
In the image forming devices such as copying machines and printers, the charging devices charges the electrostatic latent image support members such as photosensitive drums to form charged areas, which will be subjected to image exposure to form electrostatic latent images. The latent images are developed into visible images, which are transferred onto transfer members and are fixed thereon.
Various types of charging devices have been known, and can be basically classified into two types, i.e., corona charging devices utilizing corona discharging and contact charging devices in which charging brushes, charging rollers or rotary endless charging belts contact surfaces of the electrostatic latent image support members.
Although the charging device utilizing the corona discharging has an advantage that stable charging can be carried out, it causes generation of a large amount of ozone, which may deteriorate the electrostatic latent image support member and adversely affect human bodies. Therefore, attention has been given to the contact charging device because it generates remarkably less amount of ozone as compared with the corona charging device.
Especially, the attention has been given to such a brush charging device having a stationary charging brush which contacts a moving surface of the electrostatic latent image support member for charging the same because of its relatively simple structure.
In a general structure thereof, as shown in FIG. 1, a large number of brush bristles or hairs F are attached to electrically conductive base cloth A, which is fixed to back plate P of, e.g., aluminium by adhesive or the like. A brush B to which a voltage is applied by a charging power supply PS contacts a moving surface S of an electrostatic latent image support member PC to charge the surface of the support member.
However, the above brush charging device has following disadvantages. In the charging brush B, it is ideal that an upstream end B1 thereof in a moving direction X of the surface S of the latent image support member as well as opposite ends B2 of the brush in a direction Y (widthwise direction of the device) crossing the direction X are aligned or trimmed, as shown in FIG. 1. However, as shown in FIG. 2, these portions are actually liable to be disarranged such that a portion (a) of the upstream end B1 of the brush projects to the upstream side, and another portion (b) is disarranged relatively to the downstream side. Similarly, the opposite ends B2 may be disarranged in the widthwise direction Y of the device.
The disarrangement of the charging brush may often be caused by the grouping of the brush hairs due to use for a long term. The cause of the grouping is generally considered to be the mutual adhesion of the brush hairs, which may be caused by adhesion of toner component, paper dust and/or oil component in the fixing device to the brush hairs. Generally, the brush hairs are fixed by pile units each including a bundle of brush hairs, and thus it is considered that the grouping is liable to be caused in each bundle of the pile unit. Further, the grouping also may be caused by deformation of the brush hairs due to heat, pressure, vibration or the like which has been applied to the brush hairs for a long term.
If the charging brush B is disarranged in the moving direction X of the surface of the electrostatic latent image support member as described above, charging is caused in the convex portion (a) prior to the charging in the concave portion (b) by the Paschen's law. Due to this influence, a portion corresponding to the concave portion (b) around the portion (a) is charged to some extent. Therefore, in the concave portion (b), a potential difference between the charging brush B and the electrostatic latent image support member cannot reach a discharge threshold voltage according to the Patchen's law, and thus the discharging is not caused. Consequently, irregular charging is caused in the widthwise direction Y of the electrostatic latent image support member, and dark and light noise of a striped form is generated in the finally obtained image due to the irregularly charged pattern.
Also, the disarrangement of the brush at the opposite ends B2 in the widthwise direction Y of the charging device may correspondingly cause the striped noise in the image. The brush having the disarranged ends B2 are liable to contact the portions, which do not bear photosensitive layers and are located at ends in the widthwise direction of the electrostatic latent image support member. This may cause escape and drop of the voltage. In order to prevent this, the area bearing the photosensitive material must be sufficiently large in the widthwise direction, which increases the manufacturing cost of the electrostatic latent image support member.
The Japanese Laid-Open Patent Publication No. 61-105566 (105566/1986) has taught a support plate, which serves as means for preventing the toppling or inclining of the charging brush hairs and supports the downstream end of the charging brush in the moving direction of the surface of the electrostatic latent image support member. This support member, however, is aimed to prevent smear of the charging brush, as described in the publication, and cannot prevent the striped noise in the image, to be solved by the present invention.
Further, the above problem relating to the striped noise may often caused if the brush hairs have circular cross sections, because such brush hairs contact the electrostatic latent image support member through an extremely small number of points and thus effective conductive portions cannot be stably obtained. That is; the charge cannot smoothly move from the brush hairs to the electrostatic latent image support member and thus the whole surface of the support member cannot be charged uniformly and stably. The discharging is carried out only through a small number of points, so that it is difficult to perform the sufficient charging, resulting in the striped noise descried above.
The Japanese Laid-Open Patent Publication No. 62-134660 (134660/1987) has taught means for solving the above problems, in which carbon particles or the like are adhered to the surfaces of the charging brush hairs to form and disperse a large number of convex portions, i.e., conductive portions. These conductive portions are separated as the time elapses, and thus the stable charging cannot be performed for a long time of period.
The Japanese Laid-Open Patent Publication No. 3-4235 (4235/1991) has taught a construction in which hairs of a decharging brush are provided at their peripheries with uneven portions having, e.g., star-shaped cross sections. If such brush hairs were used for the charging purpose, fine powder such as fragments of toner particles, which were broken by a stress in a developing operation and/or an operation for cleaning up residual toner, would adhere to concave portions of the surfaces. This would deteriorate the charging capability of the brush hairs, resulting in reduction of the charging capability of the whole charging device. This also would cause the striped image noise due to the mixed state of the portions bearing the fine powder and the portions not bearing the same.